Coating of chromate films on surfaces as a corrosion resistance treatment for cold-rolled steel sheets, zinc-plated steel sheets and zinc-based alloy plated steel sheets, aluminum-plated steel sheets, etc. conventionally used for automobiles, household electrical appliances, construction materials and the like, is a common technique. In addition to steel sheets, chromate films are also widely used for petroleum transport pipes and other steel pipes, and for wires and other wiring materials. Aluminum and its alloy surfaces are covered with natural oxides which protect the materials from many types of corrosive environments, but anodic oxidation and chromate treatment are carried out on structural materials for aircraft and the like which require more excellent corrosion resistance and coating adhesion.
Chromate treatment often used for such metal materials includes electrolytic chromate and application-type chromate. Electrolytic chromate treatment has been carried out by cathodic electrolytic treatment of metal sheets using baths containing, for example, chromic acid as the main component, in addition to sulfuric acid, phosphoric acid, boric acid, halogens and various other anions. Application-type chromate treatment, which has been associated with the problem of elution of chromium from the chromate treated metal sheets, has been carried out by first adding an inorganic colloid or inorganic anion to a solution wherein a portion of the hexavalent chromium has been reduced to trivalent, or a solution with a specified hexavalent chromium and trivalent chromium ratio, to prepare the treatment solution, and then dipping the metal sheet therein or spraying the metal sheet with the treatment solution.
Among chromate films, those coating layers formed by electrolysis cannot be said to have sufficient corrosion resistance despite the low elution of hexavalent chromium, and their corrosion resistance is particularly low when considerable film damage occurs during working, etc. On the other hand, metal sheets coated with application-type chromate films have high corrosion resistance, and especially excellent worked portion corrosion resistance, but extensive elution of hexavalent chromium from the chromate film becomes a problem. Although elution of hexavalent chromium is controlled considerably by coating with an organic polymer, it is not sufficient. A method generally known as the resin chromate method, such as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-230666, gives an improvement in the control of elution of hexavalent chromium, but it is still impossible to avoid elution of trace amounts.
As a coating method which forms films having the same functions as conventional chromate films but containing absolutely no chromium ions, there is known a method for obtaining a corrosion resistant coating layer of a cerium-containing hydroxide while generating hydrogen gas, by dipping an Al sheet in an acidic aqueous solution of about pH 1 to 3 containing cerium ions, which method is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-502655, a double-salt film of cerium ions, zirconium ions, phosphate ions and fluorine ions on aluminum, which method is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-25579, and a zinc phosphate film formed in a zinc ion, phosphate ion and lanthanum compound treatment bath, which method is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-331658; however, none of these have adequate working-follow-up properties and sufficient corrosion resistance.
It is an object of the present invention to provide a novel technique for surface treated metal materials having corrosion-resistant coating layers with working follow-up properties and excellent corrosion resistance and employing absolutely no hexavalent chromium.